In wireless communication systems the method of adjusting transmission parameters such as modulation type, transport block size and transmission power to the current radio channel quality, is commonly referred to as link adaptation. The link adaptation is a dynamic process and serves to adapt to varying radio channel quality in order to optimize the use of transmission resources. Typically, the transmission parameters are adjusted to maintain a targeted Block Error Rate (BLER) while maximizing the data rate over the air interface. Channel-dependent scheduling of shared resources amongst users is also an important concept in many wireless communication systems to achieve as efficient resource utilization as possible. This type of scheduling strategy takes instantaneous channel quality conditions into account when allocating shared resources.
To enable link adaptation and channel-dependent scheduling, channel quality feedback is required from a receiving node, e.g. a user equipment (UE). For example, in a Wideband Code Division Multiple Access (WCDMA) system employing High-Speed Downlink Packet Access (HSDPA), fast link adaptation and scheduling of packet data may be performed on a 2 ms basis. The user equipment performs quality measurements on a Common Pilot Channel (CPICH) and derives a channel quality report, in form of a Channel Quality Indicator (CQI) value. The CQI value is reported back to the base station, for example a NodeB, and may be translated to a Signal-to-Interference Ratio (SIR) measured by the user equipment.
The CQI values may be transmitted on the High-Speed Dedicated Physical Control Channel (HS-DPCCH) in different ways. One way is to transmit the CQI values with a fixed configurable periodicity. Another possible way is to transmit the values according to uplink Dedicated Physical Control Channel (DPCCH) burst patterns during traffic inactivity if uplink discontinuous transmission from the Continuous Packet Connectivity (CPC) feature package is employed.
In general, the more frequently channel quality reports are fed back to the transmitting node, e.g. the base station,—the better knowledge that node will have of the varying channel quality experienced by the receiving node. This increases the probability of good link adaptation and also system performance when the quality reports are used to assist channel-dependent scheduling. The drawback of this is that, the reports must be signaled on the reverse link which will generate more information overhead the more frequently they are sent.
Due to delays from the time the receiving node measures the channel quality until data is scheduled for transmission to the receiving node, the experienced channel quality at the receiving node may change significantly due to e.g. fast channel fading. The experienced channel quality at transmission may be better or worse than what was reported to the transmitting node. Taking HSDPA as an example, the delay may range from approximately 7 ms up to 160 ms and may be built up of specified physical channel timing, network processing delay and delay from the time the last CQI value was fed back to the transmitting node. Consequently there is a risk of underestimating or overestimating the actual channel quality.
A known CQI adjustment strategy where a parameter CQIused is adjusted targeting a certain long-term BLER level is as follows:CQIused=CQI+CQIΔ  (1)where CQIΔ is an adaptive adjustment based on BLER measurements. Hence, CQIused may not be the same as the received CQI reported from the user equipment.
Using some kind of CQI adjustment strategy, for example an outer-loop adjustment, this should be able to handle channel variation as long as the corresponding changes are not too rapid. Typically, with low enough speed and/or an uncomplicated radio channel it will not be any problems. The CQI reporting delay may also impact accuracy.
Nevertheless, both mobile speed and propagation environment may change faster than a strategy based on BLER measurements can cope with, especially in cases with high mobile speed in a rapidly fading environment. This means that at the time of HS-DSCH transmission, the latest received CQI may be outdated.
WO 2006/075208 discloses a mechanism that applies a back-off value to the reported channel quality aiming to avoid overestimation of the real channel quality depending on the time elapsed since the quality report was received. The back-off values provides adjustments in only one direction, i.e. downwards. WO 2008/143566 addresses the facts of under- and overestimation by applying an offset value to the reported channel quality as a function of the deviation from an expected quality report value which has been generated from filtering out fast quality report variations. Both these documents use fix parameters for adjustments.
U.S. Pat. No. 7,304,939 disclose techniques for using orthogonal signals in both uplink and downlink so that simultaneous transmission in downlink and uplink in the very same frequency band can be accomplished.
A problem with existing solutions for link adaptation for downlink HSDPA is that when a specific CQI value is to be used it might be outdated due e.g. to time characteristics or variations of the radio channel in combination with the CQI reporting delay. More specifically, due to the delay, fast fading peaks and dips that have been tracked by the user equipments CPICH-measurements, may not be present at HS-DSCH transmission since channel coherence time is shorter than the total CQI delay at data transmission. Coherence time should be considered as the time over which a propagating wave could be seen as “coherent”; the time during which the phase, in average, is more or less predictable.
From this follows that the CQI estimate at time of use may be less accurate resulting in unnecessarily high BLER. Experienced channel quality may often be lower when the channel quality estimate is based on a “peak CQI” and often higher when based on a “dip CQI”, which may cause a deviation from the targeted BLER resulting in lower throughput.
If the transmission parameters are chosen based on an unreliable reported channel quality with respect to the channel coherence time, the consequences may either be underutilization, resulting in the selection of a smaller transport block than necessary, or overestimation causing block errors and thereby an inefficient use of transmission resources.
Another shortcoming of the known techniques is that the validity of the channel quality report depends on its actual age in relation to the present channel coherence time, i.e., the time it takes until the channel is uncorrelated from a point in time. If the coherence time of the channel is much longer than the total age of the quality report, the reported channel quality is statistically reliable regardless of the trend of previous quality reports. If on the other hand the communication channel is subject to large Doppler spread, equivalent to a short channel coherence time, the actual reported channel quality may be statistically uncorrelated with the experienced quality during data transmission.
Key features in evolved High Speed Packet Access (HSPA) such as CPC and uplink interference cancellation techniques may cause additional and varying channel quality reporting delays. At the same time, individual radio links may be subject to different degrees of Doppler spread. The importance may also scale with the WCDMA multi-carrier evolution, where channel qualities are reported for multiple individual carriers and channel-dependent scheduling of packet data may be done over frequencies simultaneously.